In the transmission of information from a sender to a receiver, typically a signal s representing the information at the sender is used to modulate, either in an analog or in a digital manner, the phase and/or amplitude of a purely sinusoidal carrier wave. The frequency of the carrier can also (or instead) be modulated, but for the sake of simplicity, such modulation will not be discussed further. The phase and/or amplitude-modulated carrier wave is transmitted from the sender to the receiver through a transmission medium or channel: a wire in the case of wire telephony, the atmosphere in the case of wireless (radio, microwave) telephony, an optical fiber in the case of optical telephony. In any case, the carrier in the presence or absence of signal (modulated or unmodulated carrier) arrives at the receiver where it is converted by a detector into a modulated sinusoidal electrical voltage signal v of the form v=a sin(2.pi.ft-.phi.) where a is the amplitude, f is the frequency, and .phi. is the phase of the thus detected voltage v. In a phase and/or amplitude-modulated signal, amplitude a and/or phase .phi. in general can vary as a function of time. Ideally, the variations of a and .phi. are strictly in accordance with the modulations of amplitude and phase of the carrier at the sender corresponding to the signal s. By "ideally" it is meant in the absence of noise and other disturbances. These ideal variations of a and .phi. constitute the useful signal (information) component in the voltage v at the receiver. The remaining component in v ideally represents the carrier component. A demodulator at the receiver then extracts and detects the useful signal component contained in the voltage signal v, by a process called "demodulation"; that is, a demodulator removes the carrier component from v, whereby only the signal component remains.
Ideally, therefore, when the carrier is being sent in the absence of any signal s at the sender (s=0, unmodulated carrier), the amplitude a and the phase .phi. of the voltage v detected at the receiver should both be constants. On the other hand, when a signal s is indeed present at the sender and modulates the carrier, the voltage v detected at the receiver will have a signal component corresponding to s in addition to a carrier component; that is, in the presence of signal s, either the amplitude a of the detected voltage v or the phase .phi. thereof, or both, are not constants but will vary in a manner depending upon the signal and the type of modulation being employed. In practice, however, even in the absence of any signal s at the sender, the amplitude a of the detected voltage v at the receiver unavoidably spuriously varies from time to time and place to place, owing to unavoidable variations or disturbances in the sending properties of the sender (such as fluctuations in the power level of the carrier) or owing to variations or disturbances in the transmission properties of the transmission medium or channel (such as noise). The same disturbances that produce these unavoidable spurious variations in the amplitude a of the detected voltage v at the receiver when no signal s is being transmitted also produce, when a signal s is indeed being transmitted, corresponding unavoidable spurious variations in the signal component of the amplitude a of the detected voltage v at the receiver. If adequate measures are not taken in the demodulation process to compensate for these spurious variations in amplitude a, the demodulation process will yield correspondingly spurious variations in amplitude of the extracted signal. Accordingly, workers in the art have developed automatic gain control (AGC) circuits to restore the amplitude of the carrier component of the detected voltage v to a fixed predetermined level and at the same time thereby correspondingly to restore the relative amplitude of the signal component to its ideal level, i.e., to remove the spurious variations.
In prior art, an AGC circuit typically comprises an amplifier circuit having an automatically variable gain. The detected voltage signal v is applied as input to the amplifier which amplifies the detected voltage v at the receiver with a controllably variable gain g that varies inversely with the instantaneous value of v. Accordingly, the instantaneous corresponding value of the signal strength s of the output of the amplifier, and hence also of the AGC circuit, is equal to gv. In order to achieve such a controllable gain g, a control (feedback) loop in the circuit is constructed to increase the gain g (as by decreasing the attenuation) of the AGC circuit when the instantaneous value of gv is below a desired voltage level L, and to decrease the gain g thereof (as by increasing the attenuation) when the instantaneous value of gv is above the desired level L. Thus, the AGC circuit tends to adjust gv to be equal to L.
For example, when the value of v is a steady zero (no carrier or signal), the control loop in an AGC circuit steadily increases the gain g (decreases the attenuation) until finally the gain reaches its maximum (attenuation is zero) possible value allowed by the circuit. Then, for example, starting from a moment of time when v suddenly jumps from the previous steady value of zero to a new finite steady value v.sub.1, the control loop tends to adjust the gain g such that gv.sub.1 =L. The time it takes for the control loop thus to adjust the gain g of the amplifier circuit from the maximum to the required level (L/v.sub.1) (and thus produce a steady state output) is called the "acquisition time". Conversely, starting from a moment of time later on when v suddenly jumps from v.sub.1 back to the steady value of zero, the time it takes for the control loop to (re)adjust the gain g back to the maximum value is called the "release time". As known in the art, the acquisition time is made much shorter than the release time, typically by a factor in the range of at least 10 to 100. Moreover, the period of the carrier desirably is made shorter than the release time, to prevent undesirable distortion from being introduced into the output of the AGC circuit.
Another source of undesirable distortion arises from any dc offset in the detected voltage v at the receiver or any dc offset in the AGC amplifier. The presence of such offsets, among other things, undesirably changes the gain of the AGC amplifier from its ideal value, thereby introducing spurious signals into the AGC control loop. Accordingly, precautions are ordinarily taken to reduce such offsets to negligible values.
In general, a comparator is used in the control loop of a typical AGC amplifier for continually sensing whether the instantaneous value of the amplifier output voltage gv is greater or less than the desired level L. If the amplifier output gv instantaneously goes to a value which is greater than L, then the control loop acts to decrease the gain (or increase the attenuation) of the amplifier at a relatively fast rate, determined by the acquisition time. If the AGC output gv goes less than L, then the control loop acts to increase the gain (or decrease the attenuation) of the amplifier at a relatively slow rate, determined by the release time. Accordingly, in response to a purely sinusoidal detected voltage v=a sin(2.pi.ft-.phi.), i.e., with constant a and constant .phi., ultimately the amplifier settles into a steady state, as illustrated in FIG. 1. More specifically, during those relatively long time intervals t.sub.2 t.sub.3, t.sub.4 t.sub.5, etc., when the amplifier output signal voltage strength s=gv is less than the prescribed level L, the gain g of the amplifier is being increased (attenuation is being decreased) by the control loop at a relatively slow rate; and during those relatively short time intervals t.sub.1 t.sub.2, t.sub.3 t.sub.4, t.sub.5 t.sub.6, etc., when the amplified voltage gv is greater than the prescribed level L, the gain g of the amplifier is being decreased (attenuation is being increased) by the control loop at a relatively fast rate. Thus, since the circuit is assumed to be in a steady state, the ratio of each of the mutually equal short time intervals t.sub.1 t.sub.2, t.sub.3 t.sub.4, t.sub. 5 t.sub.6, etc., to each of the mutually equal long time intervals t.sub.2 t.sub.3, t.sub.4 t.sub.5, etc., is equal to the ratio of the (short) acquisition time to the (long) release time, provided that the control loop is linear, i.e., that the rate at which the gain increases (during release) is a (relatively small) constant and the rate at which the gain decreases (during acquisition) is a (relatively large) constant. Note that as a consequence of the relative shortness of the time intervals t.sub.1 t.sub.2, t.sub.3 t.sub.4, t.sub.5 t.sub.6, etc., as compared with t.sub.2 t.sub.3, t.sub.4 t.sub.5, etc., amplitude ga of the amplifier sinusoidal v=a sin(2.pi.ft-.phi.) in the steady state will be only slightly above the level L, owing to the relatively flat profile of a sinusoid near its maximum and the resulting relatively small difference between the magnitudes of ga and L (FIG. 1). Hence the average gain g is thus only slightly greater than L/a. Moreover, in the steady state, again as a consequence of the relative shortness of the (acquisition) time intervals t.sub.1 t.sub.2, t.sub.3 t.sub.4, t.sub.5 t.sub.6, etc., during which the gain g can change rapidly (i.e., gain decreases rapidly during acquisition), the value of g in the steady state does not significantly change during any such interval t.sub.1 t.sub.2, t.sub.3 t.sub.4, t.sub.5 t.sub.6, etc. Similarly, during the relatively long (release) time intervals t.sub.2 t.sub.3, t.sub.4 t.sub.5, etc., during which the gain g changes only slowly (i.e., gain increases slowly during release), the value of g does not significantly change during any such (release) time interval. The rate of change of g is selected for the AGC amplifier such that during release the value of g does not change very much during a single period T of the carrier. Thus, the gain g of an AGC amplifier fluctuates only very slightly during the steady state, as is desired for low distortion.
Note also that if during the steady state the gain g of the AGC amplifier circuit were to increase for some reason (such as a random distribution in the control loop), so that ga would then be greater than its steady state value, then (since L is fixed) the time intervals t.sub.1 t.sub.2, t.sub.3 t.sub.4, t.sub.5 t.sub.6, etc., would suddenly increase, the intervals t.sub.2 t.sub.3, t.sub.4 t.sub.5, etc., would suddenly decrease, and the gain g would therefore experience an overall decrease during each period T because of the AGC control loop until the steady state value of ga would be restored and the steady state would thus be attained again. Similarly, in case of a sudden decrease in the gain g, the AGC amplifier would restore the steady state value of the gain. Likewise, if the amplitude a of the carrier suddenly changes, so that ga is no longer its steady state value, the AGC control loop will similarly react to restore ga back to its steady state value and thus restore the steady state of the AGC amplifier circuit. Thus, the steady state illustrated in FIG. 1 by the particular sinusoidal curve gv, with gain g such that the amplitude ga is slightly greater than L, is a stable equilibrium condition of the AGC amplifier circuit.
In a typical control loop in an AGC circuit, a comparator senses the output gv of the amplifier, compares it with the desired level L, and develops an error signal. The comparator delivers this error signal to a variable current source such that the current source delivers a relatively high predetermined positive current to, and charges up relatively quickly, a storage capacitor in response to a control loop error signal of one polarity, and the comparator delivers a relatively low predetermined negative current to, and thereby discharges relatively slowly, the same storage capacitor in response to a control loop error signal of the opposite polarity. A variable attenuator is controlled by the instantaneous voltage of the capacitor and controls the attenuation imparted to the AGC input voltage v. The charge stored in the capacitor and hence the voltage across the capacitor (relative to ground) thus determines the attenuation imparted to the input v of the AGC circuit by means of the variable attenuator which reduces the voltage delivered to the amplifier in proportion to the charge stored in the capacitor. Such a variable attenuator, for example, can be formed by the variable resistance (to ground) of an MOS transistor, whose source-drain resistance path connects the AGC input v to ground, whose gate terminal is connected to the storage capacitor, and hence whose source-drain resistance and hence attenuation is controlled by the voltage of, and hence the charge stored in, the capacitor.
On the other hand, the control loop of an AGC circuit introduces undesirable signal distortion into the amplifier output gv and hence into the demodulation process in addition to the previously mentioned distortion caused by a finite release time. In particular, signal distortions in gv are caused by the fact that various components in the control loop are nonideal (nonlinear) in their respective responses. For example, in the typical control (feedback) loop of an AGC circuit, owing to a finite resistance across the storage capacitor as needed to produce a finite release time, the charge stored therein unavoidably leaks away at a finite rate, and hence the charge that thus leaks away--for example, during each period T (FIG. 1) of a sinusoidal input signal v--must be replenished by the current source in order to maintain the steady state. Accordingly, in this steady state, the time intervals t.sub.1 t.sub.2, t.sub.3 t.sub.4, t.sub.5 t.sub.6, etc., must all be finite to accomplish this replenishing of charge, but this finite value of t.sub.1 t.sub.2, t.sub.3 t.sub.4, t.sub.5 t.sub.6, etc., results in the gain g undesirably varying by a finite amount during the course of each period T; and hence the distortion in gv likewise undesirably occurs. Moreover, the source-drain path of an MOS transistor does not have a linear source-drain current versus source-drain voltage response (at constant gate voltage), especially in case of relatively large source-drain voltage swings; therefore, the variable attenuator formed by the source-drain path of the MOS transistor introduces undesirable distortion into the (amplified) AGC output voltage gv, i.e., corresponding to the nonlinearity of the transistor's source-drain current response to its source-drain voltage. Furthermore, the control loop has a finite response time owing to various inherent delays in its various components. Therefore, when the detected voltage v changes to a different instantaneous value, the control loop does not act immediately to change correspondingly the gain g of the AGC circuit. Thus, for example, if the detected voltage v changes in an ideal case from one steady dc value a.sub.1 to another steady dc value a.sub.2, then the AGC seeks ("hunts") to adjust the gain to its proper value g=L/a.sub.2 but does not do so immediately: instead, the gain tends to oscillate around its proper value, whereby an undesirable jitter is introduced into the amplified output gv even in the ideal case of a steady dc detected voltage v. Such jitter is thus exacerbated in the practical case of constantly varying signal components in the detected voltage v.
Accordingly, it would be desirable to have an AGC circuit in which the distortions in the output caused by the AGC control loop are mitigated.